The invention relates to chemically prestressable aluminosilicate glasses having a low bubble count and bubble size. Such glasses are particularly suitable for use as substrate glasses for information recording and as safety glass sheets or moldings. After processing and prestressing, they must be able to withstand increased chemical, mechanical and thermal loads.
Thus, substrate glasses are subjected to high temperatures with short cooling times during further finishing, for example during coating in the manufacture of magnetic and optical hard disks as data storage media. When such hard disks are used, high mechanical loads occur, for example rotational speeds of 3500 to 10000 rpm and clamping stresses on the axis of rotation of up to 300 N/mm.sup.2.
Safety glass sheets are clamped in frames and doors at a pressure of at least 50 N/mm.sup.2. When destroyed mechanically, they must break into fine pieces in accordance with DIN 1249, Part 12.
Lamp covers and bulbs are subjected to large temperature gradients (.DELTA.T&gt;200.degree. C.) between the glass and the frame or holder and hot spots on the surface.
In particular, thin glasses having a thickness of from0.25 to 3.0 mm can only withstand such loads if they have been prestressed. Since increasing the mechanical strength by thermal prestressing is only possible from a minimum thickness of 3 mm, chemical prestressing by ion exchange in a salt bath is the method of choice here.
In chemical prestressing at below the glass transition temperature T.sub.g, alkali metal ions having a small ionic diameter from the glass are replaced by alkali metal ions of larger diameter from the salt bath, for example Li.sup.30 by Na.sup.+, and Na.sup.30 by K.sup.+.
Thus, with compressive stress zones having a thickness of from about 14 to 230 .mu.m, which corresponds to about 2/3 of the ion exchange depth, flexural strengths of 350 to 900 N/mm.sup.2 can be built up.
Another important factor for the above-mentioned uses is the quality of the glass with respect to the number and size of flaws, such as solid inclusions and bubbles.
Aluminosilicate glasses are known to be difficult to refine and therefore are not of high quality with respect to the lowest possible number and smallest possible size of bubbles. The glass production process produces two classes of bubble, which differ in size and have size distribution function that do not overlap. The maximum bubble diameters in the two classes are at 50 and 500 .mu.m for conventional and conventionally refined aluminosilicate glasses.
In safety glazing, all bubbles which are perceptible to the naked eye, i.e. bubbles having a diameter (.phi.) of .gtoreq.80 .mu.m, are disturbing. For this reason, quality control only accepts glass sheets which contain at most one such bubble per liter of glass volume. By contrast, bubbles which are significantly smaller, i.e. &lt; about 50 .mu.m, are not disturbing for this application. This does not apply to use as highly polished substrates for coating products, for example for hard disk substrates. Here no large bubbles may be present, and even smaller bubbles or solid inclusions having a diameter up to from 2 to 15 .mu.m are unacceptable in relatively large numbers, because, if they are at the substrate surface and are polished, they cause a hole corresponding to their diameter, which results in a loss of surfaces flatness, which makes them unsuitable for the desired application. Given the thinness of substrate glasses, the probability that a bubble present is precisely at one of the two surfaces is relatively high, as can easily be derived: for a uniform diameter D or bubbles or solid inclusions and a density N of the bubbles and inclusions, the probability W that a flaw caused by a bubble or a solid inclusion is in one of the two substrate surfaces having a size F is given by EQU W=2.times.D.times.F.times.N.
For example, for N=2500 bubbles and solid inclusions per liter of glass volume,
D=10 .mu.m, PA1 F=30 cm.sup.2,
of probability W of 0.15 is obtained. An excessively large number of bubbles of the above-mentioned small size thus also significantly reduces the production yield, by a factor of 0.85 in the example calculated above, which means that flawed substrate occurs in approximately every seventh substrate.
The poor refinability of aluminosilicate glasses can be countered with certain limits by an Li.sub.2 O content in the glass besides Na.sub.2 O. This reduces the viscosity of the glass during homogenization, which promotes degassing. Such a glass is described in DE 42 06 268 A1.
However, the presence of Li ions makes it more difficult to achieve high compressive stresses in chemical prestressing due to ion exchange, since two types of ion are exchanged, namely Li.sup.+ by Na.sup.+ and Na.sup.+ by K.sup.30 , and since a specific mixing ratio between Na and K salts and narrow temperature limits must generally be observed during the exchange process. This can result in a stress being built up only poorly, or not at all, or in the glass having no resistance to stress relaxation.
The driving force for stress relaxation in chemically prestressed glass is the concentration gradient. Together with oxygen ions, the fluoride component in the glass forms the anion network of the glass, in which large ions can easily diffuse. This favors stress reduction. Glasses having relatively high fluoride contents are thus unsuitable for chemical prestressing.